1. Field of the Invention
Embodiments of the invention relates to an organic electro-luminescent device, and more particularly, to a dual-plate organic electro-luminescent device and a method for manufacturing the same.
2. Discussion of the Related Art
Since organic electro-luminescent devices (hereinafter referred to as “organic EL devices”) in the field of flat display devices are self-luminous display devices, they have a wide viewing angle and a large contrast ratio, as compared to liquid crystal display (LCD) devices. In addition, because the organic EL devices do not require any backlight assembly, they can be light weight and have a thin profile. Further, the organic EL devices have less power consumption than LCD devices.
The organic EL devices can be driven at a low DC drive voltage and have a high response speed. Since all components of the organic EL devices are formed of solid materials, EL devices are resistant to external impact. The organic EL devices can also be used throughout a wide temperature range but yet still can be inexpensively manufactured. For example, the organic EL devices can be manufactured using deposition and encapsulation equipment. As a result, the manufacture method of the organic EL devices is very simple.
When an organic EL device is driven in an active matrix type which uses a thin film transistor arranged at each of pixel regions as a switching device for each pixel, uniform brightness can be obtained even when a low current is applied. Accordingly, the organic EL device has advantages of low power consumption, high definition and large-sized screen. Organic EL devices display a video image by exciting fluorescent materials using carriers, such as electrons and holes.
On the other hand, a passive matrix type, which has no thin film transistor for pixel switching, can also be employed to drive organic EL devices. However, the passive matrix type organic EL devices have technical limitations, such as increased power consumption and decreased lifetime. Accordingly, a variety of studies associated with active matrix type organic EL devices have been made to manufacture next-generation display devices, which meet the requirements for a high-definition large screen display.
The organic EL devices are classified into a bottom light-emitting type and a top light-emitting type, based on whether a light-emitting layer is arranged on a bottom substrate or a top substrate. In the case that the top light-emitting organic EL device is realized in an active matrix type, each thin film transistor is arranged on the bottom substrate, and the light-emitting layer is arranged on the top substrate. The organic EL device having this structure is often referred as to a “dual-plate organic electroluminescent device (DOD)”.
FIG. 1 is a cross-sectional view illustrating a structure of the related art dual-plate organic electroluminescent device. As shown in FIG. 1, a DOD includes a first substrate 10 and a second substrate 20 formed to be spaced apart from each other. The related art DOD also includes a thin film transistor array having thin film transistors (TFTs) respectively formed on the first substrate 10 in regions corresponding to sub-pixels. The related art DOD also includes an organic electroluminescent diode formed on the second substrate 20, and seal patterns 30 formed on edges of the first substrate 10 and second substrate 20. To supply a current to each organic electroluminescent diode, a conductive spacer 17 and a transparent electrode 16 are formed to connect a second electrode 25 to the TFT of an associated sub-pixel.
In each sub-pixel, the organic electroluminescent diode includes a first electrode 21 used as a common electrode and a barrier rib 26 on the first electrode 21 at the boundary between adjacent sub-pixels. The organic electroluminescent diode also includes organic electroluminescent layers 22, 23 and 24 sequentially arranged together with the second electrode 25 in a region defined by an associated barrier rib 26, namely, the associated sub-pixel, while being formed in the form of patterns separated from those of other sub-pixels.
The organic electroluminescent layers include a first carrier transporting layer 22, a light-emitting layer 23 and a second carrier transporting layer 24 laminated in sequence. The first and second carrier transporting layers 22 and 24 serve to inject and transport electrons or holes to the light-emitting layer 23. The structures of the first and second carrier transporting layers 22 and 24 are determined depending on the arrangement of an anode and a cathode. For example, the light-emitting layer 23 may be made of a high-molecular weight material, and the first carrier transporting layer 22 and the second carrier transporting layer 24 may be defined as the anode and the cathode, respectively. In this case, the first carrier transporting layer 22 contacting the first electrode 21 has a structure in which a hole injection layer and a hole transporting layer are sequentially laminated on the first electrode 21. Also, the second carrier transporting layer 24 contacting the second electrode 25 has a structure in which an electron injection layer and an electron transporting layer are sequentially laminated on the second electrode 25.
The first and the second carrier transporting layer 22 and 24, and the light-emitting layer 23 may be made of a low-molecular weight material or a high-molecular weight material. When the low-molecular weight materials are used, these layers 22, 23 and 24 are formed by a vapor deposition method. When the high-molecular weight materials are used, these layers 22, 23 and 24 are formed by an inkjet method.
A major function of the conductive spacer 17 is not to maintain a cell gap. Instead, the function of the conductive spacer 17 is to electrically connect the two substrates, in contrast to the typical spacers for liquid crystal display (LCD) devices. Thus, the conductive spacer 17 has a certain three-dimensional shape in a space defined between the two substrates.
Each TFT connected to the associated organic electroluminescent diode is a driving thin film transistor. The TFT includes a gate electrode 11 formed at a predetermined portion on the first substrate 10, a semiconductor layer 13 formed in an island shape to cover the gate electrode 11, and a source electrode 14a and a drain electrode 14b formed at opposite sides of the semiconductor layer 13. A gate insulating film 12 is formed over the entire upper surface of the first substrate 10 between the gate electrode 11 and the semiconductor layer 13 in each sub-pixel. A passivation film 15 is formed on the gate insulating film 12, including the source electrode 14a and the drain electrode 14b. The drain electrode 14b is electrically connected to the transparent electrode 16 formed on the passivation film 15 through a hole disposed in the passivation film 15. The conductive spacer 17 is in contact with the top of the transparent electrode 16.
The conductive spacer 17 serves to electrically connect the drain electrode 14b of the TFT arranged at the associated sub-pixel on the first substrate 10 to the second electrode 25 arranged on the second substrate 20. The conductive spacer 17 is a metal-coated column spacer formed of an organic insulating material and the like. When the portions of the first substrate 10 corresponding to the sub-pixels are joined to corresponding portions of the second substrate 20, the conductive spacer 17 allows an electric current to flow between the drain electrode 14b and the second electrode 25 in the associated sub-pixel.
The outer portion of the conductive spacer 17 is made of a conductive metal material. In this case, a metal having high ductility is used. Further, the metal should have low resistivity.
The first electrode 21 is made of a transparent material. The second electrode 25 is formed of a light-shielding metal layer. The space between the first electrode 21 and the second electrode 25 is filled with an inert gas or insulating liquid.
Although not shown in FIG. 1, storage capacitors, scanning lines, signal lines and power supply lines crossing the scanning lines are formed on the first substrate 10.
The DOD includes a bus line having a lattice structure formed on the first electrode 21 made of a transparent material having a high specific resistivity. The bus line contributes to the prevention of a voltage drop in the first electrode 21. This will be explained in reference to FIGS. 2 and 3 below.
FIG. 2 is a plan view illustrating a bus line of the related art dual-plate organic electroluminescent device. FIG. 3 is a plan view illustrating a bus line and a barrier rib in one unit pixel of FIG. 2. With reference to FIGS. 2 and 3, the related art DOD also includes a bus line 50 formed in the form of a lattice in regions other than pixel regions on the first electrode 21, as shown in FIG. 2. The first electrode 21 is made of a transparent material to prevent a reduction in aperture ratio. However, when a voltage is applied to the first electrode 21, a drop in the voltage applied may occur in a center part of a display region due to a high specific resistivity of the transparent electrode material. To prevent such a voltage drop, the bus line 50, which has a low specific resistivity, is further provided in the related art DOD.
A barrier rib 55 is formed just on outside portion of the bus line 50 (i.e., a region other than pixel regions) and also overhangs the bus line 50 with an undercut structure so as to define a region where an organic electro-luminescent layer is to be formed. During formation of the second electrode on the electro-luminescent layer, such an undercut structure allows the barrier rib 55 to prevent a short circuit between the second electrode and the bus line. In other words, the undercut structure of the barrier rib 55 maintains an insulative separation between the second electrode and the bus line.
FIG. 4 is an SEM view illustrating a state in which the undercut of the barrier rib collapses in regions indicated by the circled reference numerals in FIG. 3. FIG. 5 is an SEM view illustrating a normal state of the undercut of the barrier rib in other regions other than regions indicated by the circled reference numerals in FIG. 3. Corner portions of the undercut of the barrier rib, as indicated by the circled reference numerals, partially overlap the bus line, as shown in FIG. 4. In these regions, the barrier rib has under portions which are not on the bus line but have protrusions overlapping the bus line. As a result, the protrusions of the undercut, which have a small thickness, may collapse toward the bus line.
On the other hand, in the remaining portions of the undercuts, since the barrier rib is formed on the outside portion of the bus line, it is located in regions where the bus line has no step, as shown in FIG. 5. In this region, accordingly, no protrusion of the barrier rib undercut comes in contact with the bus line therebeneath. As shown in FIG. 4, when the protrusions of the barrier rib collapses toward the bus line, the second electrode is not only formed on the organic light-emitting layer, including the barrier rib, but also on a portion of the bus line being in contact with the collapsed undercut. As a result, the first electrode and the second electrode may be short-circuited. Accordingly, normal operation cannot be achieved in regions where an undercut collapse has occurred.
When the barrier rib has an undercut structure, corner portions of the undercut of the barrier rib, such as the regions indicated by the circled reference numerals, are formed over the bus line. As a result, the protrusions of the undercut, which have a small thickness, may collapse down toward the bus line. In this case, the second electrode is not formed on the organic light-emitting layer including the barrier rib, but formed on a portion of the bus line being in contact with the collapsed undercut. As a result, the first electrode and the second electrode may be short-circuited. Accordingly, normal operation cannot be achieved in regions where an undercut collapse has occurred onto the bus line. In particular, regions where there is an undercut structure crosses a step of the bus line, the protrusion can be three times larger than a protrusion of the undercut located in other regions where there is no step crossing the bus line. The larger protrusions of the undercut become thinner so that undercut collapse may occur.